Most mining operations on alluvial or placer type deposits use "wet" gravity separation technology. That is, particles of gold and other heavy metals are separated from lighter weight dust and dirt by using a current of water. Although wet plant technology is usually sufficiently efficient to render mining operations profitable, it requires a large supply of water and also generates a large amount of debris. Consequently, wet gravity separators cannot be used in dry areas. Furthermore, dams are often required to be built to contain the environmentally hazardous slimes created by dirty water, adding to the cost of production. For environmental reasons, the use of large-scale wet separation plants is restricted or even prohibited in some countries.
To overcome such problems, air separators or concentrators, sometimes called "dry blowers", have been used for waterless recovery of gold and other heavy minerals from dry placer deposits. There are several categories of air concentrators or dry blowers, but most involve passing air through the deposit as it is conveyed along the concentrator to thereby separate heavy particles from lighter material. Apparatus such as bellows, fans and compressors are typically used to vary air flow through the deposit being processed, while vibrating feed devices, angled shaking tables and screens, gravity bypass methods, air tunnel devices and perforating moving belts are used to convey the deposit through the concentrator. Examples of known air concentrators or separators can be found in U.S. Pat. Nos. 2,752,041; 3,105,040; 3,080,056; 3,108,950; 3,207,306; 3,367,502; 4,294,693; 4,615,797; 4,642,180 and Australian patent application no. 14534/83.
Many concentrators, such as those described in U.S. Pat. Nos. 2,752,041; 4,615,797 and 4,642,180, are limited to small scale batch operations and are of limited application. Others may be suited to continuous or larger scale operations, but are invariably of expensive and complex construction with many moving parts, and generally require vibratory mechanisms to achieve high throughput of deposit. Such concentrators are not easily transported. As a result of these disadvantages, the known air concentrators have failed to find widespread user acceptance.
A common design feature of known air concentrators is that the deposit is processed as it passes along a generally linear path in single direction, e.g. down an inclined riffle board or longitudinally along a table. Consequently, known concentrators are generally of elongated configuration, and their throughput is limited by their maximum dimension.
Perhaps the most significant disadvantage of most known air concentrators or dry blowers is their low efficiency, i.e. poor concentration rates and relatively low recovery rates. Since, for any given deposit, the average grade of ore is generally constant, and as the price of gold and other minerals are fixed by the market, the only remaining variable which will determine whether a particular deposit is viable is production cost which, in turn, is directly dependant upon efficiency. Many deposits therefore, are not viable with known air separation plants of low efficiency.
It is an object of the present invention to overcome at least some of the disadvantages of prior art air separation plants by providing a relatively low cost, high volume, environmentally compatible, efficient air concentrator.